This invention relates to a navigation system for guiding a driver of a motor vehicle when the vehicle approaches an intersection on a predetermined route. More particularly, this invention relates to such a navigation system in which misleading guidance of the driver at an intersection due to erroneous detection of vehicle position is prevented.
A conventional vehicle navigation system is provided with a display screen such as a cathode ray tube CRT, liquid crystal display, or other display means for displaying a road map thereon. The route and the current position of the vehicle are indicated on the displayed road map. When the vehicle approaches an intersection of a predetermined route, the driver is informed by a signal on the screen or by an audible message.
Such a navigation system continually detects the current position of the vehicle and computes the distance between the current position and a next intersection on the predetermined route. When the distance becomes equal to or less than a given value, the system informs the driver that the vehicle is approaching the next intersection. Therefore, if the current position as detected is incorrect due to a detection error, the system may incorrectly inform the driver that the vehicle is approaching the intersection when the vehicle is not actually approaching the intersection.
In this case, since the driver associates the signal with an intersection on the route indicated on the displayed road map, the location or direction in which he perceives the vehicle is heading may be incorrect. Resultantly, the driver may turn right at an intersection where he should actually go straight, or go straight at an intersection where he should turn right, etc.
There is known another type of navigation system by which a driver is informed of the direction to take at the intersection the vehicle is approaching by an audible message. In this type of system, the driver takes, at the intersection, the direction as informed without paying attention to the route indicated on the displayed road map. Accordingly, even if the vehicle is deviating at the intersection from the predetermined route, the driver can recognize the deviation only long after it takes place, resulting in a large deviation from the determined route.